Polymerization process

ABSTRACT

Polymerization process including polymerizing a monomer and a comonomer in a polymerization reaction, withdrawing an effluent stream containing solid polymer and a mixture of unreacted monomer and unreacted comonomer, and passing the effluent to a high pressure recovery system having (a) a high pressure separation step for separating vapor containing unreacted monomer and unreacted comonomer from the solids, and (b) a high pressure recycle system for recycling a portion of the vapor to the polymerization reaction, passing the solids from the high pressure recovery system to a low pressure recovery system having (a) a low pressure separation step for separating further unreacted monomer and unreacted comonomer from the solids, and (b) a low pressure recycle system for recycling at least a portion of the unreacted monomer and unreacted comonomer to the polymerization reaction. A portion of the vapor separated in step 2(a) is passed to the low pressure recovery system.

This application is the U.S. national phase of International ApplicationNo. PCT/EP2013/054645 filed Mar. 7, 2013 which designated the U.S. andclaims priority to European Patent Application Nos. 12159940.1, filedMar. 16, 2012, Ser. No. 12159942.7, filed Mar. 16, 2012, and Ser. No.12159944.3, filed Mar. 16, 2012, the entire contents of each of whichare hereby incorporated by reference.

The present invention relates to the treatment and recycle of effluentstreams from a polymerisation process.

BACKGROUND OF THE INVENTION

The production of polymer powder by polymerisation reactions of monomersin the presence of catalysts is well-known. For example, processes areknown and widely operated commercially using both fluidised bed reactorsand slurry phase reactors.

In a slurry polymerisation process, for example, the polymerisation isconducted in a stirred tank or, preferably, a continuous loop reactor inwhich a slurry of polymer particles in a liquid medium comprisinghydrocarbon diluent is circulated. During the course of polymerisation,fresh polymer is generated by the catalytic polymerisation of monomerand polymer product is removed from the reactor by removing a portion ofthe slurry.

The slurry withdrawn may be treated to separate the polymer particlesfrom the hydrocarbon diluent and other components, such as unreactedmonomers, which it is generally desired are recycled to the process.

A slurry polymerisation process generally includes feed systems forfresh monomer and comonomer, as well as for fresh inert liquids. Freshfeeds of monomer and comonomer for example are fed to the polymerisationprocess to replace monomer and comonomer consumed in the reaction.Although inert liquids don't react they can be lost from the system inprocess purges or as residual amounts in the polymer passed todownstream storage and processing.

Process purges are required in the system to remove undesired inertcomponents and poisons, which otherwise build up to detrimental levelsin the process. Examples include inert hydrocarbons corresponding to thedesired monomers and co-monomers. For example, where ethylene is used asa monomer ethane may be present as an impurity in the ethylene feed andcan also be produced in the reaction by hydrogenation of ethylene. Otherhydrocarbons can be present even where the corresponding monomers arenot used. For example methane and propane are often present in lowlevels in ethylene, and propane can also be present at low levels inisobutane.

It is desired to maintain low levels of such components, although ingeneral the lower the level that is maintained the more other componentsare lost in the purges. Thus, purging is usually operated to try andmaintain a balance in the system between levels of undesired compoundsand losses of desired compounds.

The purges are usually applied during the effluent treatment steps in apolymerisation process. In particular, polymer withdrawn from a slurrypolymerisation reaction is removed in slurry form in a liquid mediumcomprising inert diluent, quantities of unreacted monomer and comonomer,and components such as impurities and hydrogen. It is desired to recoverthe polymer solids essentially free of the other components, and torecycle as much as possible the useful hydrocarbons to thepolymerisation reaction.

A common method to achieve this in slurry polymerisation is to heat thewithdrawn slurry to vaporise the liquid medium, and to separate thevapour from the polymer solids. This is generally referred to as a“flash”. The vapours can then be condensed and recycled to the reaction,whilst the polymer solids can be recovered for further treatment.

It has become conventional that a first separation step is performed ata relatively high pressure, for example a high pressure flash step. Thepolymer solids are then usually let down in pressure to a lower pressuresecond separation step, which may be a lower pressure flash step or aflush step (in which a gas is contacted with the polymer to removeremaining hydrocarbons from the polymer) and remaining hydrocarbons arethereby removed from the polymer.

The pressure and temperature in the high pressure first separation stepare generally selected such that the majority of the diluent, monomerand comonomer are recovered in the vapour, and said vapour can becondensed without compression for recycle to the reactor.

The hydrocarbons removed in the lower pressure second step are stillpresent in sufficient quantities that it is economic to recover andrecycle them to the process. However, the low pressure second separationstep, in contrast to the high pressure recovery system, generally leadsto recovered components, such as diluent, monomer and comonomer, whichmust be compressed (or further cooled) in order to be able to condensethem prior to recycle to the reactor.

(“Compression” refers to a process of increasing the pressure(“compressing”) a gas or mixture of gases. This is a relatively energyintensive process. Once in the form of liquids, liquids can be pumped toincreased pressure with relatively less difficulty. Avoiding“compression”, for example by condensing without compression, is highlydesirable.)

Examples of such systems can be found, for example, in WO 2005/003188which discloses the use of a higher pressure flash stage followed by alower pressure flush stage. However, processes are also known where thelower pressure stage is a flash stage rather than a flush stage, orwhere both flashing and flushing occur in a single stage. (It can benoted that a flush stage can also be referred to as a “purge stage”. Theterm “flush” is used herein for such steps to avoid any confusion withprocess purges, which are steps whereby streams are removed from apolymerisation process, for example to flare. The term “purge” as usedherein therefore refers to a stream which is removed from the processrather than a flush step.)

Treatments which may be applied to one or both of the separated streamsprior to recycle, or at least portions thereof, include treatments toseparate components such as “heavy” hydrocarbons and “light”hydrocarbons. The separated heavy and light hydrocarbons are generallypurged, usually to flare.

As noted above, the use of a high pressure separation step minimises thecompression required to recycle the separated vapours, and it isgenerally desired that as much of the liquid medium is recovered in thisstep as possible. Removal of significantly in excess of 90% of theliquid medium is obtainable.

SUMMARY OF THE INVENTION

We have now surprisingly found that the treatment process can beadvantageously operated by deliberately letting down in pressure aportion of a recovered high pressure recycle stream, and passing this toa low pressure treatment system.

Thus, in a first aspect, there is provided a polymerisation processcomprising the steps of:

-   -   1) Polymerising a monomer and a comonomer in a polymerisation        reaction,    -   2) Withdrawing an effluent stream comprising solid polymer and a        mixture comprising unreacted monomer and unreacted comonomer,        and passing the effluent to a high pressure recovery system        comprising        -   a. a high pressure separation step for separating a vapour            comprising unreacted monomer and unreacted comonomer from            said solids, and        -   b. a high pressure recycle system for recycling a portion of            the vapour to the polymerisation reaction,    -   3) Passing the solids from the high pressure recovery system to        a low pressure recovery system comprising        -   a. a low pressure separation step for separating further            unreacted monomer and unreacted comonomer from said solids,            and        -   b. a low pressure recycle system for recycling at least a            portion of the unreacted monomer and unreacted comonomer to            the polymerisation reaction,            characterised in that a portion of the vapour separated in            step 2(a) is passed to the low pressure recovery system.

The polymerisation process is preferably a slurry polymerisationprocess, in which case the polymerisation process may comprise the stepsof:

-   -   1) Polymerising a monomer and a comonomer in the presence of a        diluent in a polymerisation reaction,    -   2) Withdrawing an effluent stream comprising solid polymer and a        mixture comprising diluent, unreacted monomer and unreacted        comonomer, and passing the effluent to a high pressure recovery        system comprising        -   a. a high pressure separation step for separating a vapour            comprising diluent, unreacted monomer and unreacted            comonomer from said solids, and        -   b. a high pressure recycle system for recycling a portion of            the vapour to the polymerisation reaction,    -   3) Passing the solids from the high pressure recovery system to        a low pressure recovery system comprising        -   a. a low pressure separation step for separating further            diluent, unreacted monomer and unreacted comonomer from said            solids, and        -   b. a low pressure recycle system for recycling at least a            portion of the further diluent, unreacted monomer and            unreacted comonomer.

DETAILED DESCRIPTION OF THE INVENTION

In general, the terms “high pressure” and “low pressure” are used hereinto indicate the relative pressures of two systems rather than theabsolute pressures. The pressure differential between the high and lowpressure recovery systems is generally at least 0.2 MPa (2 bar),preferably at least 0.4 MPa (4 bar).

As noted previously, the pressure and temperature in the high pressureseparation step/high pressure recovery system are generally selectedsuch that the majority of the diluent, monomer and comonomer arerecovered in the vapour, and said vapour can be condensed withoutcompression for recycle to the reactor.

In this context, “high pressure” generally refers to streams and stageswhich are at a pressure of 0.5 MPa (5 bar) and above. Usually thepressure is 0.7 MPa (7 bar) and above. There is no specific maximumpressure, but for practical purposes the term “high pressure” is usuallyless 2 MPa (20 bar), and usually less than 1.5 MPa (15 bar).

The low pressure recovery system, in contrast to the high pressurerecovery system, leads to recovered components, such as diluent, monomerand comonomer, at significantly lower pressures.

In this context, “low pressure” generally refers to streams and stageswhich are at a pressure of less than 0.5 MPa (5 bar), usually less than0.4 MPa (4 bar). Although pressures less than atmospheric pressure arepossible, the “low pressure” systems are usually at a pressure of atleast 0.1 MPa (1 bar).

For avoidance of any doubt, unless otherwise indicated, values ofpressure as quoted herein are “absolute” values rather than “gauge”values.

The portion of the vapour separated in step 2(a) which is passed to thelow pressure recovery system generally comprises at least 0.5% by weightof the vapour separated in step 2(a), such as at least 1% by weight orat least 5% by weight of the vapour separated in step 2(a).

The portion of the vapour separated in step 2(a) which is passed to thelow pressure recovery system usually comprises at least 10% by weight ofthe vapour separated in step 2(a), preferably at least 20% by weight,such as between 20 and 40 wt % and most preferably 20 to 30 wt % of thevapour separated in step 2(a).

The majority of the vapour separated in step 2(a) is recycled to thepolymerisation reaction whilst maintaining pressure at or above 0.5 MPa(5 bar), preferably at or above 0.7 MPa (7 bar). The stream may thus beconsidered as a high pressure recycle stream.

The portion recycled to the polymerisation reaction preferably comprisesthe majority of the vapour from step 2(a), and most preferably at least60%, such as at least 70% of the vapour separated in step 2(a).

Portions may however be taken from this stream to treatment steps, forexample to produce an olefin-free diluent stream.

In particular, all or a portion may be cooled and taken to a (first)separator from which at least a portion of the light components otherthan monomer are removed to leave a condensed liquid recycle stream. Allor a portion of the light components removed may be purged from theprocess as a purge stream, preferably to flare.

The term “separator” as used herein means a process unit in whichseparation of gas and liquid streams can occur. Examples of “separators”include gas/liquid separation vessels and fractionation columns.

In particular, the first separator is preferably a “high pressureseparator” by which is meant a separator operated at a pressure of 0.5MPa (5 bar) and above. Preferably, the separator is operated at apressure of 0.7 MPa (7 bar) and above.

In general terms, the term “lights” as used herein means propane andmolecules having a molecular weight less than propane. The portion ofthe light components other than monomer which may be removed in theseparator in the present invention generally comprises light componentssuch hydrogen, nitrogen and methane. The first separator may thereforebe considered as a first “lights separator”, by which as used herein ismeant a separator which is operated to provide a separation of hydrogen,nitrogen and methane from monomer and heavier components present. Thegeneral concept of “lights separators” for separation of lightcomponents in polymerisation processes is well-known (along with“heavies separators” for separation of “heavy” components). One exampleof such a system is taught by U.S. Pat. No. 6,292,191 although in thisdocument the lights column is operated to remove hydrogen, oxygen,nitrogen and ethylene from diluent to give a purified, olefin-free,diluent stream, whereas in the present invention it is desired tomaintain monomer in the liquid stream.

The vapour comprising light components recovered from the firstseparator is preferably further cooled to less than −10° C., whilstmaintaining pressure at 0.5 MPa (5 bar) and above, preferably 0.7 MPa (7bar) and above. This may then be passed to a further separator in thehigh pressure recovery system. Condensed liquid from the furtherseparator is recycled to the first separator, whilst overhead vapour ispassed to flare. This stream is referred to as the high pressure flarestream.

In general, light components, such as hydrogen, methane and nitrogen canbe separated with very high specificity into the high pressure flarestream, and in particular at least 75% of each of such components fed tothe further separator are passed into the high pressure flare stream. Asa particular example, at the temperature and pressure of the separationstep the vapour stream typically comprises over 95%, of the hydrogen fedto the further separator.

Similarly, any heavy components, such as 1-hexene and hexane wherepresent, can separate with very high specificity into the liquid streamexiting the first separator, by which is meant greater than 95% of eachof such components fed to the first separator are recovered in theliquid stream.

According to the present invention a portion of the vapour separated inthe high pressure recovery system is passed to the low pressure recoverysystem.

For avoidance of doubt, although it should be clear from the above, thereference to passing “a portion of the vapour separated in step 2(a)” tothe low pressure recovery system does not mean that the separated vapourremains in the vapour state after separation in step 2(a). Inparticular, it is preferred that the vapour separated in the highpressure recovery system is condensed, and then a portion thereof isseparated, let-down in pressure and then passed to the low pressurerecovery system.

More preferably, the portion of the vapour separated in the highpressure recovery system which is passed to the low pressure recoverysystem is a portion separated from the liquid stream recovered from thefirst separator described above.

In general, letting down a portion of the recovered vapour/condensedliquid would be expected to be disadvantageous because any recoveredstreams let-down in pressure have to be recompressed to be re-used. Toavoid the requirement for this as much as possible is exactly the reasonwhy high pressure separation systems are used to try to maximise highpressure recovery of reaction components in the first place.

Surprisingly, however, it has been found that passing a portion of thevapour recovered at high pressure to the low pressure recovery systemresults in improvements in the overall separations, and in particular inthe ability to purge undesired components from the high pressure and lowpressure recovery systems with reduced loss of useful components.

In particular, it is believed that the portion of the vapour separatedin the high pressure recovery system is relatively unsaturated inmonomer. When let down to the lower pressure and passed to the lowpressure recovery system there is an increase in the overall recovery ofmonomer for recycle.

The additional recovery of monomer leads to a reduction in monomerlosses in the process purges. Whilst monomer efficiencies ofpolymerisation processes, which is calculated herein as the amount ofmonomer fed which is not purged, are generally very high (above 98.5%),at the scale of commercial polymerisation processes even what appears asrelatively minor increases in efficiency can result in significant costsavings, as well as significant reductions in hydrocarbon emissions orcombustion products from hydrocarbon emissions (when flared). Forexample, in a process producing 50 tonnes/hour of polymer, an increasein monomer efficiency by only 0.1% is still a reduction in monomerlosses of 50 kg/hour.

More particularly, the process of the present invention is able toprovide a monomer efficiency in excess of 99.5%, for example of 99.6%and above, and most preferably of 99.7% and above.

The monomer efficiency is a measure of the amount of the monomer whichends up in the polymer product, and is determined from the amount offresh monomer fed to a process and the amount of monomer which ispurged. The monomer purge rate may be determined from the purge flow andthe concentration of monomer in the purge stream, which can be measuredby GC, for each purge stream present. The efficiency may be determinedinstantaneously, based on flow rate measurements at a particular time,but preferably is determined over a period of time, for example based onaveraged instantaneous measurements or on total amounts fed and purgeddetermined over a period of at least several hours, as this generallygives a more accurate measurement. The monomer efficiency is determinedby subtracting the amount purged from the amount fed, and then dividingthe result by the amount fed. This answer is multiplied by 100 to givethe efficiency as a percentage.

Similarly, the process of the present invention can provide an increasein comonomer efficiency, which as used herein as the amount of comonomerfed which is not purged. More particularly the process of the presentinvention is able to provide a comonomer efficiency in excess of 95%,for example of 97% and above, and most preferably of 98% and above.

The portion of the vapour separated in step 2(a) which is passed to thelow pressure recovery system is preferably passed to the low pressurerecycle system part of the low pressure recovery system. Put anotherway, the portion of the vapour separated in step 2(a) is preferably notpassed to the low pressure separation step (step 3(a)) for separatingfurther unreacted monomer and unreacted comonomer from the solids.

The portion of the vapour separated in step 2(a) which is passed to thelow pressure recovery system is preferably passed to a low pressure andlow temperature separator in the low pressure recovery system (alsoreferred to herein as a “second separator”). In particular, the presentinvention preferably comprises a low pressure recovery system whichcomprises a second separator which is at a pressure of less than 0.4 MPa(4 bar) and at a temperature of less than −10° C. (The second separatoris a part of the low pressure recovery system.)

The second separator may also be considered as a second “lightsseparator”, which is operated to provide a separation of light compoundssuch as hydrogen, nitrogen and methane as already defined.

The majority of light components other than monomer, such as nitrogen,hydrogen and methane are again recovered as an overhead vapour stream. Aportion of this stream is purged from the system, preferably to a flare.

Although the first and second separators generally result in hydrogenpassing to the overhead streams, and hence to any purge streams, thequantities of hydrogen in the process is generally small, and thehydrogen is more cost effectively flared than recycled and recovered tothe overall polymerisation process. In fact, a further advantage of thepresent invention is that the purging of hydrogen is efficient enoughthat other impurities which can be present in fresh hydrogen feeds, suchas methane and CO, can also be efficiently purged from the system viathe purge streams, and the separate purification of fresh hydrogen feedto remove such components can be reduced or avoided.

Thus, a low hydrogen efficiency of a polymerisation process has alsobeen found to be advantageous.

The process of the present invention preferably has a hydrogenefficiency, measured as the amount of the fed hydrogen which is notpurged of 80% or less, preferably of 70% or less, and most preferably of60% or less.

The hydrogen efficiency may be determined in a similar manner to themonomer efficiency, and in particular by determining the amount ofhydrogen purged from the purge flow and the hydrogen concentration inthe purge stream, which can be measured by GC, for each purge streampresent and comparing this to the amount of hydrogen fed to the process.

The combination of the first and second separators described above isparticularly preferred. Thus, in a preferred embodiment the presentinvention provides a polymerisation process comprising the steps of:

-   -   1) Polymerising a monomer and a comonomer in a polymerisation        reaction,    -   2) Withdrawing an effluent stream comprising solid polymer and a        mixture comprising unreacted monomer and unreacted comonomer,        and passing the effluent to a high pressure recovery system        comprising        -   a. a high pressure separation step for separating a first            vapour stream comprising unreacted monomer and unreacted            comonomer from said solids, and        -   b. a high pressure recycle system for recycling a portion of            the first vapour stream to the polymerisation reaction, said            high pressure recycle system comprising            -   i. a first separator at a pressure greater than 0.5 MPa                (5 bar) for separating a second vapour stream comprising                at least a portion of light components other than                monomer in the first vapour stream and a liquid stream                comprising condensed portions of the first vapour                stream,    -   3) Passing the solids from the high pressure recovery system to        a low pressure recovery system comprising        -   a. a low pressure separation step for separating further            unreacted monomer and unreacted comonomer from said solids,            and        -   b. a low pressure recycle system for recycling at least a            portion of the further unreacted monomer and unreacted            comonomer to the polymerisation reaction, said low pressure            recycle system comprising            -   i. a second separator which operates at a pressure of                less than 0.5 MPa (5 bar), preferably less than 0.4 MPa                (4 bar), and at a temperature of less than −10° C.                characterised in that a portion of the liquid stream                from step 2(b)(i) is passed to the second separator.

Further, the present invention provides a process for enhancing theseparation of monomer components from light components other thanmonomer, which process comprises:

-   -   A) Providing a first stream comprising monomer and light        components other than monomer    -   B) Passing the first stream to a first separator at a first        pressure to separate a second stream comprising at least some of        the light components other than monomer and provide a third        stream comprising monomer,    -   C) Passing a portion of the third stream and a fourth stream        comprising monomer and light components other than monomer to a        second separator at a second pressure which is lower than the        first pressure to separate a fifth stream comprising at least        some of the light components other than monomer from the fourth        stream and provide a sixth stream comprising monomer.

In this aspect of the present invention the first stream comprisingmonomer and light components other than monomer is passed to a firstseparator to separate at least some of the light components other thanmonomer therein, and then a portion of the recovered third stream isreduced in pressure and passed with a fourth stream also comprisingmonomer and light components other than monomer to a second separator.

The process of this aspect of the invention results in a reduction inthe concentration of monomer in the combined second and fifth streamscompared to a process where the first and fourth streams are passedseparately to the first and second separators without passing a portionof the third stream to the second separator.

The first pressure is higher than the second pressure. Preferably thefirst pressure is at least 0.5 MPa (5 bar). Preferably the secondpressure is less than 0.5 MPa (5 bar), and more preferably is less than0.4 MPa (4 bar). Preferably the pressure difference between the firstand second pressures is at least 0.2 MPa (2 bar), more preferably atleast 0.3 MPa (3 bar).

The second separator is preferably at a pressure of less than 0.4 MPa (4bar) and a temperature of less than 10° C.

The monomer in the process of the present invention is preferably anolefin monomer. For avoidance of any doubt, the term “monomer” as usedherein refers to the olefin which is present in the largest amount inthe formed polymer, and may also be referred to as the “principalmonomer”, whilst the term “comonomer” as used herein refers to olefinsother than the monomer which may be present. More than one comonomer maybe present.

The monomer is preferably ethylene or propylene, most preferablyethylene.

Where ethylene is the monomer, propylene may be the comonomer, but thecomonomer is preferably selected from 1-butene, 1-hexene and 1-octene,with 1-hexene being most preferred.

Where propylene is the monomer, the comonomer is preferably selectedfrom ethylene, 1-butene, 1-hexene and 1-octene.

The comonomer is preferably 1-hexene.

Preferred diluents which may be used are inert hydrocarbons, morepreferably butanes, especially iso-butane, pentanes and mixturesthereof. Iso-butane is most preferred.

In a preferred embodiment of the present invention there are also passedto the “low pressure and low temperature separator” in the low pressurerecovery system one or more fresh liquid feeds for the polymerisationreaction.

“Fresh” as used herein means a component which is being passed to thereaction for the first time and can be contrasted with “recycle” streamswhich contain components recovered from the reaction effluent forrecycle. However, for avoidance of doubt such streams may have beensubjected to pre-treatments to reduce impurities.

The fresh liquids which are passed to a slurry polymerisation reactioninclude the diluent in the process. Such components are added to thereaction and form part of the liquid medium of the slurry in thereaction. As noted above, examples of such liquids are inerthydrocarbons, most preferred diluents including butanes, especiallyiso-butane, pentanes and mixtures thereof. Iso-butane is most preferred.

The fresh liquids to a slurry polymerisation reaction can also includecomonomers. As noted above, commonly used comonomers include 1-butene,1-hexene and 1-octene, although other comonomers can be used.

It is preferred that both fresh comonomer and fresh diluent are passedto the second separator.

It has been surprisingly found that passing fresh diluent and/or freshcomonomer to the second separator also improves the efficiency of theoverall process in removing impurities whilst minimising losses ofmonomer and diluent.

Although it is possible to also feed fresh comonomer and/or freshmonomer directly to the polymerisation reaction, it is preferred in thisembodiment that the majority of fresh comonomer passed to the reactionis passed via the claimed process and that the majority of fresh diluentpassed to the reaction is passed via the claimed process i.e. via thesecond separator and most preferably via the second separator at apressure of less than 0.4 MPa (4 bar) and at a temperature of less than−10° C.

Most preferably, it is preferred that all of the fresh comonomer passedto the reaction is passed via the claimed process and all of the freshdiluent passed to the reaction is passed via the claimed process

The process of the present invention may be applied to a polymerisationprocess operating in one or more reactors. The operation of two loopreactors in series, for example, is well-known. The term “polymerisationreaction” as used herein is intended generically to refer topolymerisation occurring in a single reactor or polymerisation occurringin two or more reactors.

Thus, in a system comprising two or more reactors, a recycle system forrecycling to “the polymerisation reaction” may recycle components to asingle reactor or to both reactors in the system.

Where at least two reactors are operated in parallel then effluentstream from each may be withdrawn, combined and passed to a commonrecovery system (e.g. to a single high pressure recovery system and asubsequent single low pressure recovery system).

Where at least two reactors are operated in series, then an effluentstream is usually withdrawn from the last reactor in series for passageto the high pressure recovery system (and then to the low pressurerecovery system. Effluent withdrawn from an earlier reactor in series isgenerally passed to a subsequent reactor in the series, althoughintermediate treatments are possible, for example to remove hydrogen orcomonomer. (Such treatments may, in fact, be required depending on theparticular process being operated.)

EXAMPLES General Process

Ethylene is polymerised in two slurry loop reactors in series to producea bimodal polyethylene with a density of 948 kg/m³ and a Melt Index(MI₅) of 0.31.

In the first reactor ethylene is polymerised in the substantial absenceof comonomer, but in the presence of hydrogen and with isobutane asdiluent. Polymer from the first reactor is passed to a second reactorwherein further ethylene is polymerised in the presence of 1-hexene ascomonomer and the substantial absence of hydrogen, again in the presenceof isobutane.

Polymer slurry is withdrawn from the first reactor and passed to acontacting vessel in the form of a stirred tank.

Polymer slurry is recovered from the base of the contacting vessel andpassed to the second reactor.

Polymer slurry is withdrawn from the second reactor and passed via aslurry heater, in which the liquid components of the slurry arevaporised, to a flash tank at a pressure of 0.95 MPa.

Polymer solids are withdrawn from the flash tank for further processing.

The vapour recovered from the flash tank is passed as the first vapourto the contacting vessel where it is contacted with the slurry withdrawnfrom the first reactor.

Vapour is withdrawn from the top of the contacting tank and passed to afractionator in which it is contacted with a reflux stream. Liquidsrecovered from the base of the fractionator are returned to thecontacting tank.

The combined “contacting tank/fractionator” is herein referred to as a“fractionation system”.

Vapour recovered overhead from the fractionator is cooled and partiallycondensed, then passed to a liquid/vapour (first) separator operating ata temperature of 35° C. and a pressure of 0.91 MPa.

A portion of the liquid recovered therefrom is utilised as the refluxstream to the fractionator.

The vapour recovered from the liquid/vapour separator is further cooledand separated at −35° C. and 0.91 MPa in a further separator in the highpressure recovery system. Condensed liquid is recycled to theliquid/vapour first separator, whilst overhead vapour is passed toflare. This stream is referred to as the high pressure flare stream.

Polymer solids withdrawn from the flash tank are passed for furtherprocessing in a flush vessel at a pressure of 0.135 MPa. The flushingtakes place by contact in two vertically orientated stages with polymerbeing introduced at the top and withdrawn from the base of the vessel,and with a recycled flush gas being introduced into the upper stage andnitrogen being introduced into the lower stage.

A mixture of the flush gases and recovered diluent/monomer is recoveredfrom the top of the flush vessel, cooled and passed to low pressuretreatment steps. A portion is passed to a heavies separation unit toremove heavy components, but the majority (including a recycle streamfrom the heavies separation unit) is passed to a (second) separator at apressure of 0.40 MPa and at a temperature of −30° C.

From the base of the second separator is recovered a liquid stream whichcan be recompressed and recycled to the second reactor.

Overhead from the second separator is recovered a vapour stream. Themajority of this stream is recycled to the flush vessel, but a portionis passed to a flare. This stream is referred to as the low pressureflare stream.

COMPARATIVE EXAMPLE

In a Comparative Example a portion of the condensed liquid recovered inthe high pressure recovery system from the liquid/vapour separator (at atemperature of 35° C. and a pressure of 0.91 MPa) is taken and treated,but the remainder is pumped back to a higher pressure and recycled tothe first reactor.

The composition of the high pressure and low pressure flare streams areshown in Table 1

EXAMPLE ACCORDING TO THE INVENTION

In this Example a portion of the condensed liquid recovered in the highpressure recovery system from the liquid/vapour first separator (at atemperature of 35° C. and a pressure of 0.91 MPa) is taken and treatedas in the Comparative Example. A further portion is let-down in pressureand fed to the low pressure recovery system, and in particular iscombined with the majority of the stream recovered from the flush vesselwhich is passed to the second separator (at a pressure of 0.40 MPa andat a temperature of −30° C.). The reminder of the condensed liquidrecovered in the high pressure recovery system from the liquid/vapourseparator is pumped back to a higher pressure and recycled to the firstreactor as in the Comparative Example. In order to balance the overallprocess the amount taken and utilised as the reflux stream to thefractionator is reduced.

The further portion passed to the low pressure recovery system comprisesabout 27% of the condensed liquid recovered from the liquid/vapour firstseparator.

The composition of the high pressure and low pressure flare streams areshown in Table 2.

TABLE 1 High pressure Low pressure flare stream flare stream Component(kg/h) (kg/h) Total Nitrogen 21.40 268.09 289.52 Ethylene 293.03 16.90309.93 Ethane 94.46 2.62 97.08 Propane 1.96 0.66 2.63 Iso-butane 39.9263.43 103.35 1-hexene 0.84 0.84 Hexane 0.03 0.03 Methane 15.87 0.33 16.2Hydrogen 12.89 0.02 12.91

TABLE 2 High pressure Low pressure flare stream flare stream Component(kg/h) (kg/h) Total Nitrogen 61.55 227.96 289.51 Ethylene 162.67 50.57213.24 Ethane 69.58 27.42 97.00 Propane 1.40 1.55 2.94 Iso-butane 35.3784.84 120.22 1-hexene 0.16 0.16 Hexane 0.01 0.01 Methane 13.86 2.3016.16 Hydrogen 12.12 0.52 12.64

Comparing the results in Tables 1 and 2, similar amounts of nitrogen,ethane and hydrogen are purged overall, but the total ethylene losses inthe purges are reduced by 31% (from 310 kg/h to 213 kg/hr).

The reduction in the total ethylene losses in the Example compared tothe Comparative Example corresponds to a monomer efficiency increasefrom 99.3 to 99.6%.

The losses of 1-hexene are also reduced, in particular from 0.84 to 0.16kg/hr. The reduction in the total 1-hexene losses Example compared tothe Comparative Example corresponds to a comonomer efficiency increasefrom about 95 to about 99%.

The invention claimed is:
 1. A polymerisation process comprising thesteps of: 1) Polymerising a monomer and a comonomer in a polymerisationreaction, 2) Withdrawing an effluent stream comprising solid polymer anda mixture comprising unreacted monomer and unreacted comonomer, andpassing the effluent to a high pressure recovery system, said highpressure recovery system being at a pressure of at least 0.5 MPa, andcomprising a. a high pressure separation step for separating a vapourcomprising unreacted monomer and unreacted comonomer from said solids,and b. a high pressure recycle system for recycling a portion of thevapour to the polymerisation reaction, 3) Passing the solids from thehigh pressure recovery system to a low pressure recovery system, saidlow pressure recovery system being at a pressure of less than 0.5 MPa,and comprising a. a low pressure separation step for separating furtherunreacted monomer and unreacted comonomer from said solids, and b. a lowpressure recycle system for recycling at least a portion of theunreacted monomer and unreacted comonomer to the polymerisationreaction, wherein the majority of the vapour separated in step 2(a) isrecycled as a high pressure recycle stream to the polymerizationreaction while maintaining pressure at or above 0.5 MPa (5 bar), andwherein a portion of the vapour separated in step 2(a) is passed to thelow pressure recovery system by deliberately letting down in pressure aportion of the recovered high pressure recycle stream and passing thisto the low pressure treatment system.
 2. A process according to claim 1wherein the polymerisation process is a slurry polymerisation process,and steps (1) to (3) are: 1) Polymerising a monomer and a comonomer inthe presence of a diluent in a polymerisation reaction, 2) Withdrawingan effluent stream comprising solid polymer and a mixture comprisingdiluent, unreacted monomer and unreacted comonomer, and passing theeffluent to a high pressure recovery system comprising a. a highpressure separation step for separating a vapour comprising diluent,unreacted monomer and unreacted comonomer from said solids, and b. ahigh pressure recycle system for recycling a portion of the vapour tothe polymerisation reaction, 3) Passing the solids from the highpressure recovery system to a low pressure recovery system comprising a.a low pressure separation step for separating further diluent, unreactedmonomer and unreacted comonomer from said solids, and b. a low pressurerecycle system for recycling at least a portion of the further diluent,unreacted monomer and unreacted comonomer.
 3. A process according toclaim 1 wherein the pressure differential between the high pressurerecovery system and the low pressure recovery system is at least 0.2 MPa(2 bar).
 4. A process according to claim 1 wherein the high pressurerecovery system is at a pressure of at least 0.7 MPa (7 bar).
 5. Aprocess according to claim 1 wherein the low pressure recovery system isat a pressure of less than 0.4 MPa (4 bar).
 6. A process according toclaim 1 wherein the portion of the vapour separated in step 2(a) whichis passed to the low pressure recovery system comprises at least 1% byweight of the vapour separated in step 2(a).
 7. A process according toclaim 1 wherein the portion of the vapour separated in step 2(a) whichis passed to the low pressure recovery system comprises between 20 and40 wt % of the vapour separated in step 2(a).
 8. A process according toclaim 1 wherein a portion of the recycle stream of step 2(b) iscondensed and taken to a first separator from which at least a portionof the light components are removed overhead and from the base of whichis removed a liquid stream, and the portion of the vapour separated instep 2(a) which is passed to the low pressure recovery system is aportion of this liquid stream.
 9. A process according to claim 1 whereinthe portion of the vapour separated in step 2(a) which is passed to thelow pressure recovery system is passed to the low pressure recyclesystem part of the low pressure recovery system and not passed to thelow pressure separation step (step 3(a)) for separating furtherunreacted monomer and unreacted comonomer from the solids.
 10. A processaccording claim 1 wherein the portion of the vapour separated in step2(a) which is passed to the low pressure recovery system is passed to aseparator (second separator) in the low pressure recovery systemoperated at a pressure of less than 0.4 MPa (4 bar) and at a temperatureof less than −10° C.
 11. A process according to claim 10 wherein aportion of the overhead stream from the separator in the low pressurerecovery system is purged from the system.
 12. A process according toclaim 1 wherein there are also passed to the separator in the lowpressure recovery system one or more fresh liquid feeds for thepolymerisation reaction.
 13. A process according to claim 12 whereinboth fresh comonomer and fresh diluent are passed to the separator inthe low pressure recovery system.
 14. A process according to claim 1wherein the polymerisation reaction takes place in two or more reactorsand the recycle systems for recycling to the polymerisation reaction mayrecycle components to a single reactor or to both reactors in thesystem.
 15. A process according to claim 1 which has a monomerefficiency in excess of 99.5%.